Although the production and use of technical hexachlorocyclohexane (HCH) and lindane (the purified insecticidal isomer γ-HCH) are prohibited in most countries, residual concentrations still constitute an immense environmental burden. Many studies describe the mineralization of γ-HCH by bacterial strains under aerobic conditions. However, the metabolic fate of the other HCH isomers is not well known. In this study, we investigated the transformation of α-, β-, γ-, δ-, ε-HCH, and a heptachlorocyclohexane isomer in the presence of varying ratios of the two enzymes that initiate γ-HCH degradation, a dehydrochlorinase (LinA) and a haloalkane dehalogenase (LinB). Each substrate yielded a unique metabolic profile that was strongly dependent on the enzyme ratio. Comparison of these results to those ofin vivoexperiments with different bacterial isolates showed that HCH transformation in the tested strains was highly optimized towards productive metabolism of γ-HCH and that under these conditions other HCH-isomers were metabolized to mixtures of dehydrochlorinated and hydroxylated side-products. In view of these results, bioremediation efforts need very careful planning and toxicities of accumulating metabolites need to be evaluated.